Aqueous paint formulations containing mineral fillers generally consist of an aqueous phase, of one or more polymers in emulsion in the liquid phase referred to as binders. fillers and/or pigments, a dispersant and admixtures as diverse as surfactants, coalescent agents, biocides, and anti-foaming agents, and finally, at least one thickening agent.
The latter allows a control of the rheology of aqueous formulations in which it is introduced, and in particular in aqueous paints, both at the manufacturing stage as well as during their transport, storage, or in the course of their implementation. The diversity of practical constraints at the level of each of these steps reflects a multiplicity of different rheological behaviours.
One can nevertheless summarize the need of the person skilled in the art to obtain a thickening effect in the aqueous formulation, both for reasons of stability over time as well as for a possible application of paint to a vertical surface, the absence of coating splash at the time of implementation, etc. This is why the additives which contribute to this control of rheological behaviour are known as thickeners.
Among these products, there are the so-called “associative” thickeners which are water-soluble polymers with insoluble hydrophobic groups. Such macro-molecules have an associative character: once introduced into water, the hydrophobic groups are susceptible to association in the form of micellar aggregates. These aggregates are linked to one another by the hydrophilic parts of the polymers: there is then the formation of a three-dimensional network that causes the increase in the viscosity of the medium.
The operating mechanism and characteristics of associative thickeners are well known today and are described for example in the documents “Rheology modifiers for water-borne paints” (Surface Coatings Australia, 1985, pp. 6-10) and “Rheological modifiers for water-based paints: the most flexible tools for your formulations” (Eurocoat 97, UATCM, vol. 1, pp 423-442).
Among these associative thickeners, there is the class of associative thickeners of the HEUR (Hydrophobically modified Ethylene oxide URethane) type. They designate copolymers resulting from the synthesis between a compound of the polyalkylene glycol type, a polyisocyanate and a monomer or condensate called “associative” of the alkyl, aryl or aryalkyl type consisting of a hydrophobic terminal group.
These structures are well known for developing high viscosities for an average to low shear gradient (j. of Applied Polymer Science, vol. 58, p 209-230, 1995; Polymeric Mat. Sci. and Engineering, vol. 59, p 1033, 1988; Polymeric Mat. Sci. and Engineering, vol. 61, p 533, 1989; Polymeric Paint Colour Journal, vol. 176, No. 4169, p 459, June 1986), which corresponds respectively to Stormer™ (KU) and Brookfield™ (mPa·s) measurements of viscosity.
But it is a more complex rheology that we are seeking, a real compromise between the application properties of the paint and its behaviour in the can. On the one hand, we aim to increase the Stormer™ viscosity of the formulation: this results in a better stability of the paint in storage and an improved performance when the product is agitated (appearance in the can) and is picked up with the application tool (better loading of the roller, paintbrush or brush). On the other hand, the increase in the Brookfield™ viscosity must be limited in a concomitant manner: such an increase results in a degradation of the application properties of the paint (limitation of levelling, presence of coating splashes).
To this double problem, the associative polyurethane thickeners market provided a solution in the 1990s which constitutes a reference: Acrysol™ SCT-275 (DOW™) However, this product contains alkyl phenols. Now these substances are today widely suspected of being carcinogenic and dangerous to reproduction. Still tolerated in the paint industry, they nonetheless remain of interest to legislative institutions, particularly those of Europe.
There is therefore a real need to develop an associative thickener of the HEUR type in order to resolve the double technical problem referred to above without using alkyl phenols. In order to do so, the person skilled in the art has a very broad library of structures that differ essentially in the choice of the hydrophobic monomer.
Document EP 1 566 393 describes a HEUR type thickener, one of the essential characteristic of which is the presence of n-butyl-1-octanol, while its hydrophobic groups are based on fatty alcohols with 8 to 18 carbon atoms. Document DE 10 206 023001 describes an associative non-ionic thickening agent of the HEUR type including a branched linear alcohol. Document EP 1 241 198 describes polyurethane-based thickening agents including a monoalcohol having 6 to 22 carbon atoms. Document EP 1 013 264 describes a polyurethane thickener for cosmetic formulations with an associative monomer functionalized by a hydrophobic group that can be linear or branched, but preferentially linear and possessing 12 to 24 carbon atoms. Document WO 94/06840 proposes an associative thickener of the HEUR type characterized by a certain density of hydrophobic groups, the said groups being linear alkyl chains with 8 to 22 carbon atoms. Document EP 1 584 331 proposes a hydrophobic terminal group with 6 to 34 carbon atoms for the associative monomer. To specifically increase the Brookfield™ viscosity, document EP 0 639 595 proposes linear hydrophobic groups having 4 to 36 carbon atoms. Document WO 02/102868 also makes reference to linear structures for the associative monomer.
However, outside of the alkyl phenols, none of the hydrophobic structures envisaged until now provides satisfaction for increasing the Stormer™ viscosity while limiting the increase in the Brookfield™ viscosity, and thus achieving an optimal compromise between stability on the one hand, the can appearance, the tool loading, and on the other hand the application properties such as levelling and the absence of coating splash.